Ion source having different modes of operation

ABSTRACT

An ion source that is capable of different modes of operation is disclosed. A vaporizer is in communication with the ion source. The ion source may have several gas inlets, in communication with different gasses. When operating in a first mode, the ion source may supply a first gas, such as an inert gas, while heating the vaporizer. When operating in a second mode, the ion source may supply a second gas, which may be an organoaluminium gas. When operating in a third mode, the ion source may supply the second gas, while heating the vaporizer. Ions having single charges may be created in the first and second modes, while ions having multiple charges may be created in the third mode.

FIELD

Embodiments of the present disclosure relate to an ion source and more particularly, an ion source having multiple modes to generate ions of a species having different charges.

BACKGROUND

Various types of ion sources may be used to create the ions that are used in semiconductor processing equipment. For example, an indirectly heated cathode (IHC) ion source operates by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the arc chamber of the ion source. The cathode is disposed at one end of an arc chamber. A repeller may be disposed on the end of the arc chamber opposite the cathode. The cathode and repeller may be biased so as to repel the electrons, directing them back toward the center of the arc chamber. In some embodiments, a magnetic field is used to further confine the electrons within the arc chamber. A plurality of sides is used to connect the two ends of the arc chamber.

An extraction aperture is disposed along one of these sides, proximate the center of the arc chamber, through which the ions created in the arc chamber may be extracted.

In certain embodiments, it may be desirable to create ions that have a single charge. However, in other embodiments, it may be desirable to create ions that are multicharged. Unfortunately, for certain materials, such as aluminum and other metals, the mechanisms that are used to create singly charged ions may not be effective in creating multicharged ions. Therefore, different ion sources may be utilized depending on the desired charge of the extracted ions.

This solution is expensive as it utilizes several ion sources. Further, this solution is time consuming, as it takes time to switch from one ion source to a different ion source.

Therefore, a single ion source that is capable of operating in different modes in order to generate ions having different charges would be beneficial. Additionally, it would be advantageous if the arc chamber could be changed from one mode to another quickly.

SUMMARY

An ion source that is capable of different modes of operation is disclosed. A vaporizer is in communication with the ion source. The ion source may have several gas inlets, in communication with different gasses. When operating in a first mode, the ion source may supply a first gas, such as an inert gas, while heating the vaporizer. When operating in a second mode, the ion source may supply a second gas, which may be an organoaluminium gas. When operating in a third mode, the ion source may supply the second gas, while heating the vaporizer. Ions having single charges may be created in the first and second modes, while ions having multiple charges may be created in more abundance in the third mode.

According to one embodiment, an indirectly heated cathode ion source is disclosed. The indirectly heated cathode ion source comprises an arc chamber, comprising a plurality of walls; an indirectly heated cathode disposed in the arc chamber; a vaporizer in communication with the arc chamber; a heater to heat dopant material disposed within the vaporizer; a first valve in communication with the arc chamber and a first gas source; a second valve in communication with the arc chamber and a second gas source; and a controller in communication with the first valve, the second valve and the heater so as to operate the indirectly heated cathode ion source in one of a plurality of modes. In some embodiments, the plurality of modes comprises two single charge modes to create ions of a species having a single charge and a multicharge mode to create ions of the species having two or more charges. In some embodiments, the species comprises a metal. In some embodiments, in a first single charge mode, the controller actuates the heater, opens the first valve and closes the second valve. In some embodiments, in a second single charge mode, the controller disables the heater, and opens the second valve. In certain embodiments, in the second single charge mode, the controller opens the first valve. In some embodiments, in the multicharge mode, the controller actuates the heater, and opens the second valve. In some embodiments, the dopant material disposed in the vaporizer comprises a solid compound comprising a metal and the metal is also a component of a second gas contained in the second gas source. In certain embodiments, the metal is aluminum and the dopant material disposed in the vaporizer is aluminum chloride. In certain embodiments, the metal is aluminum and the dopant material disposed in the vaporizer is aluminum iodide. In some embodiments, the metal is aluminum and the first gas source contains an inert gas and the second gas source contains dimethylaluminum chloride or trimethylaluminum chloride.

According to another embodiment, a method of operating an indirectly heated cathode ion source in a plurality of modes, wherein the indirectly heated cathode ion source comprises a controller, an arc chamber, a vaporizer in communication with the arc chamber, and a heater to heat dopant material in the vaporizer, is disclosed. The method comprises selecting a desired mode of operation; and using the controller to configure the indirectly heated cathode ion source to operate in the desired mode, wherein to operate in a multicharge mode, wherein the multicharge mode is used to create ions of a species having two or more charges, the heater is actuated so that vaporized dopant material comprising a metal enters the arc chamber and the controller enables a flow of a gas containing the metal into the arc chamber; wherein in a first single charge mode, wherein the first single charge mode is used to create ions of the species having a single charge, the controller enables the heater and enables the flow of an inert gas into the arc chamber; and wherein to operate in a second single charge mode, wherein the second single charge mode is used to create ions of the species having a single charge, the controller disables the heater and uses the gas containing the metal to generate a plasma. In some embodiments, the metal is aluminum and the dopant material disposed in the vaporizer is aluminum chloride. In some embodiments, the metal is aluminum and the dopant material disposed in the vaporizer is aluminum iodide. In some embodiments, the metal is aluminum and the gas containing the metal comprises DMAC or TMAC.

According to another embodiment, an indirectly heated cathode ion source is disclosed. The indirectly heated cathode ion source comprises an arc chamber, comprising a plurality of walls; a vaporizer in communication with the arc chamber; an indirectly heated cathode disposed in the arc chamber, wherein the indirectly heated cathode is used to generate a plasma in the arc chamber; and a controller configured to operate the indirectly heated cathode ion source in one of a plurality of modes, wherein in multicharge mode, the controller configures the indirectly heated cathode ion source such that two sources of a metal are used in a generation of a plasma. In some embodiments, the metal is aluminum, material disposed in the vaporizer comprises aluminum chloride and wherein a gas containing aluminum is introduced into the arc chamber when operating in the multicharge mode. In some embodiments, the metal is aluminum, material disposed in the vaporizer comprises aluminum iodide and wherein a gas containing aluminum is introduced into the arc chamber when operating in the multicharge mode. In certain embodiments, in a single charge mode, only one of the two sources is introduced into the arc chamber.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

FIG. 1 is an indirectly heated cathode (IHC) ion source having several modes of operation in accordance with one embodiment; and

FIG. 2 shows the operation of the controller using the embodiment of FIG. 1 .

DETAILED DESCRIPTION

As noted above, certain dopants, such as aluminum and other metals, utilize different mechanisms to create singly charged ions and multicharged ions.

FIG. 1 shows an IHC ion source 10 that overcomes these issues. The IHC ion source 10 includes an arc chamber 100, comprising two opposite ends, and walls 101 connecting to these ends. The walls 101 of the arc chamber 100 may be constructed of an electrically conductive material and may be in electrical communication with one another. In some embodiments, a liner may be disposed proximate one or more of the walls 101. A cathode 110 is disposed in the arc chamber 100 at a first end 104 of the arc chamber 100. A filament 160 is disposed behind the cathode 110. The filament 160 is in communication with a filament power supply 165. The filament power supply 165 is configured to pass a current through the filament 160, such that the filament 160 emits thermionic electrons. Cathode bias power supply 115 biases filament 160 negatively relative to the cathode 110, so these thermionic electrons are accelerated from the filament 160 toward the cathode 110 and heat the cathode 110 when they strike the back surface of cathode 110. The cathode bias power supply 115 may bias the filament 160 so that it has a voltage that is between, for example, 200V to 1500V more negative than the voltage of the cathode 110. The cathode 110 then emits thermionic electrons on its front surface into arc chamber 100.

Thus, the filament power supply 165 supplies a current to the filament 160. The cathode bias power supply 115 biases the filament 160 so that it is more negative than the cathode 110, so that electrons are attracted toward the cathode 110 from the filament 160. In certain embodiments, the cathode 110 may be biased relative to the arc chamber 100, such as by bias power supply 111. In other embodiments, the cathode 110 may be electrically connected to the arc chamber 100, so as to be at the same voltage as the walls 101 of the arc chamber 100. In these embodiments, bias power supply 111 may not be employed and the cathode 110 may be electrically connected to the walls 101 of the arc chamber 100. In certain embodiments, the arc chamber 100 is connected to electrical ground.

On the second end 105, which is opposite the first end 104, a repeller 120 may be disposed. The repeller 120 may be biased relative to the arc chamber 100 by means of a repeller bias power supply 123. In other embodiments, the repeller 120 may be electrically connected to the arc chamber 100, so as to be at the same voltage as the walls 101 of the arc chamber 100. In these embodiments, repeller bias power supply 123 may not be employed and the repeller 120 may be electrically connected to the walls 101 of the arc chamber 100. In still other embodiments, a repeller 120 is not employed.

The cathode 110 and the repeller 120 are each made of an electrically conductive material, such as a metal or graphite.

In certain embodiments, a magnetic field is generated in the arc chamber 100. This magnetic field is intended to confine the electrons along one direction. The magnetic field typically runs parallel to the walls 101 from the first end 104 to the second end 105. For example, electrons may be confined in a column that is parallel to the direction from the cathode 110 to the repeller 120 (i.e. the y direction). Thus, electrons do not experience any electromagnetic force to move in the y direction. However, movement of the electrons in other directions may experience an electromagnetic force.

Disposed on one side of the arc chamber 100, referred to as the extraction plate 103, may be an extraction aperture 140. In FIG. 1 , the extraction aperture 140 is disposed on a side that is parallel to the Y-Z plane (perpendicular to the page).

Further, the IHC ion source 10 may be in communication with at least two gas sources. The first gas source 170 may contain a first gas, which may be an inert gas, such as argon. A first valve 171 may be utilized to control the flow of the first gas from the first gas source 170 to the IHC ion source 10. The second gas source 175 may contain a second gas that is an organoaluminium compound, which is a compound in which an aluminum atom is bonded with a carbon atom. In certain embodiments, the organoaluminium compound contains a halogen and aluminum. In certain embodiments, this second gas may be dimethylaluminum chloride (DMAC; (CH₃)₂AlCl) or trimethylaluminum chloride (TMAC; (CH₃)₃AlCl). Other gases that include a metal atom bonded to a carbon atom may also be used. In some embodiments, this second gas comprises carbon, a metal and a halogen. The second gas source 175 may also include various diluent gasses, such as hydrogen, argon or other gasses. In other words, the second gas source 175 contains the second gas, but may also include other gasses. A second valve 176 may be utilized to control the flow of the second gas from the second gas source 175 to the ion source 10. The first valve 171 and the second valve 176 may be mass flow controllers (MFC) such that the flow rate may be controlled.

A vaporizer 190 may be in communication with the arc chamber 100. For example, the vaporizer may be disposed outside the arc chamber 100, but may include a conduit 191 connecting the output of the vaporizer with the arc chamber 100. A heater 195 may be disposed proximate the vaporizer 190 to heat and vaporize the dopant material 197 disposed within the vaporizer 190. The heater 195 may be a resistive heater or another type. The design of the heater is implementation specific and not limited by this disclosure. In certain embodiments, the dopant material 197 within the vaporizer 190 may be a solid compound that contains a metal. For example, the dopant material 197 may be aluminum chloride or aluminum iodide. In other embodiments, the dopant material 197 within the vaporizer 190 may be gallium iodide, indium iodide, aluminum iodide, lanthanum fluoride or another solid compound containing a metal and a halogen. The metal included in the dopant material 197 is the same metal contained in the second gas.

A controller 180 may be in communication with one or more of the power supplies such that the voltage or current supplied by these power supplies may be modified. The controller 180 may also be in communication with the first valve 171, the second valve 176 and the heater 195. The controller 180 may include a processing unit, such as a microcontroller, a personal computer, a special purpose controller, or another suitable processing unit. The controller 180 may also include a non-transitory storage element, such as a semiconductor memory, a magnetic memory, or another suitable memory. This non-transitory storage element may contain instructions and other data that allows the controller 180 to perform the functions described herein.

In the embodiment shown in FIG. 1 , the controller 180 is configured to allow the ion source 10 to operate in a plurality of different modes. These modes include a first single charge operating mode; a second single charge operating mode; and a multicharge operating mode. Each of these modes will be described in more detail.

In the first single charge mode, the filament power supply 165 passes a current through the filament 160, which causes the filament 160 to emit thermionic electrons. These electrons strike the back surface of the cathode 110, which may be more positive than the filament 160, causing the cathode 110 to heat, which in turn causes the cathode 110 to emit electrons into the arc chamber 100. These electrons collide with the molecules of gas that are fed into the arc chamber 100 through the gas inlet.

The controller 180 opens the first valve 171 so as to allow the flow of the first gas into the arc chamber 100. The first gas may be an inert gas. The controller 180 also controls the heater 195 to as to heat the dopant material 197 within the vaporizer 190 to a temperature such that the dopant material 197 is vaporized and vaporized dopant material enters the arc chamber 100 through the conduit 191. The combination of electrons from the cathode 110, the first gas and the vaporized dopant material creates a plasma. The ions in this plasma may be mostly single charged ions, such as Al⁺. In certain embodiments, the electrons and positive ions may be somewhat confined by a magnetic field. In certain embodiments, the plasma is confined near the center of the arc chamber 100, proximate the extraction aperture 140. Thus, in this first single charge mode, the metal is introduced only by the vaporized dopant material.

Thus, when it is desired to create single charged ions, such as Al⁺, the operator may transmit this preference to the controller 180. Alternatively, the controller 180 may determine the desired mode based on the desired charge state and beam current. In response, the controller 180 may perform the sequence described above.

In the second single charge mode, the controller opens the second valve 176 as to allow the flow of the second gas, which is a gas that comprises the same metal as the dopant material 197, into the arc chamber 100. The controller 180 also disables the heater 195 such that no vapor from the vaporizer 190 enters the arc chamber 100. The second gas is ionized, producing metal ions having mostly single charges. Further, if the second gas contains a halogen, the halogen in the second gas serves to recycle metal from the walls of the arc chamber 100. Thus, in this second single charge mode, the metal is introduced only by the second gas. In certain embodiments, the first valve 171 may be open to allow the flow of the first gas into the arc chamber 100. In other embodiments, the first valve 171 is closed.

In multicharge mode, the controller 180 opens the second valve 176 so as to allow the flow of the second gas into the arc chamber 100. As noted above, the second gas may be a gas that comprises the same metal as the dopant material 197. For example, if the metal is aluminum, the second gas may be DMAC or TMAC. These gasses are useful in that they effectively recycle metal from the walls of the arc chamber 100. The controller also enables the heater 195 such that vaporized dopant material from the vaporizer 190 also enters the arc chamber 100 via the conduit 191. As described above, the metal in the vaporized dopant material is the same metal that is a component of the second gas. Because the second gas and the vaporized dopant material both contain the same metal, a plasma that is rich in this metal is created. Due to the density of the metal in the plasma, many of the ions that are created in the plasma may be multicharged ions, such as Al⁺⁺ or Al⁺⁺⁺. The ions can then be extracted through the extraction aperture 140 and used as an ion beam.

In certain embodiments, the controller 180 may also open the first valve 171 to allow the flow of some first gas into the arc chamber 100. Thus, in the enhanced mode, the second valve 176 is opened and the first valve 171 may be open or closed.

Thus, when it is desired to create multicharged ions, the operator may transmit this preference to the controller 180. Alternatively, the controller 180 may determine the desired mode based on the desired charge state and beam current. In response, the controller 180 may perform the sequence described above.

Thus, with respect to the embodiment shown in FIG. 1 , the present application describes three different modes of operation that may be used to generate different charge states of the desired dopant. Further, by incorporating the first gas, the second gas and a vaporizer 190 in communication with the arc chamber 100, the ion source 10 can easily switch from one mode to another mode with no operator intervention. FIG. 2 shows the operation of the controller 180 to control the modes of the ion source 10 when a vaporizer 190 is in communication with the arc chamber 100. As shown in Box 200, the desired mode of operation is selected, which may depend on the recipe that is being used. This mode may be selected by the operator or user. Alternatively, the controller 180 may automatically select the most appropriate mode based on the desired beam current and charge state. Based on this selection, the controller 180 manipulates the first valve 171, the second valve 176 and the heater 195 to achieve the desired mode of operation. Typically, since the dopant material charge in the vaporizer 190 is limited, the second gas source would be used for most Al⁺ operation in order to conserve the dopant material in the vaporizer 190 and effectively extend source life.

As shown in Box 210, the multicharge mode may be selected, where plasma conditions are optimized for the generation of multicharged metal ions. In response, the controller 180 opens the second valve 176 to allow the flow of the second gas, which is a gas comprising the same metal as the dopant material 197, into the arc chamber 100. The controller 180 may optionally open or close the first valve 171. The controller 180 also enables the heater 195 such that vaporized dopant material from the vaporizer 190 also enters the arc chamber 100 through the conduit 191. This vaporized dopant material contains the same metal as the second gas. Since there are two sources of the metal, the plasma is rich with metal ions. This metal-rich plasma is effective at creating multicharged ions and can achieve multicharged ion currents in excess of using either metal source independently.

Alternatively, as shown in Box 220, the first single charge mode may be selected, where the majority of metal ions have a single charge. In response, the controller 180 opens the first valve 171 to allow the flow of the first gas, which may be an inert gas such as argon, into the arc chamber 100. In this mode, the controller 180 enables the heater 195 such that vaporized dopant material enters the arc chamber 100 through the conduit 191. The inert gas is ionized with the vaporized dopant material in a plasma, where the majority of the ions that are created are single charged ions.

As shown in Box 230, the second single charge mode may be selected, where the majority of metal ions have a single charge. In the second single charge mode, the controller 180 disables the heater 195 so that vaporized dopant material does not enter the arc chamber 100. Further, in this mode, the controller controls the second valve 176 to allow the flow of the second gas into the arc chamber 100. The second gas is ionized, producing metal ions having mostly single charges. In some embodiments, the first valve 171 is also opened to allow the flow of the first gas into the arc chamber 100. In other embodiments, the first valve is closed. Further, the halogen in the second gas serves to recycle metal from the walls of the arc chamber 100. Thus, in this second single charge mode, the metal is introduced only by the second gas.

Thus, to operate in the multicharge mode, the controller 180 configures the ion source 10 such that the metal is introduced by the second gas and the vaporized dopant material. To operate in the single charge mode, the controller 180 configures the ion source 10 such that the metal is introduced by only one of the second gas or the vaporized dopant material.

While the above disclosure describes the use of a dopant material that contains aluminum as the solid compound in the vaporizer 190 and a second gas that contains aluminum, other metals may be used as well. For example, the metal may be gallium. In this embodiment, solid compound in the vaporizer 190 may be gallium iodide, and the second gas may be an organogallium gas. In another embodiment, the metal may be indium. In this embodiment, the solid compound in the vaporizer 190 may be indium iodide, and the second gas may be an organoindium gas. In another embodiment, the metal may be lanthanum. In this embodiment, the solid compound in the vaporizer 190 may be lanthanum fluoride, and the second gas may be an organolanthanide gas. Other metals may be used as well.

The embodiments described above in the present application may have many advantages. First, the creation of an ion source that can operate in a plurality of modes is advantageous, as the same ion source may be used to create single charged ions and multicharged ions. Additionally, the combination of a vaporizer with a solid metal compound and a gas that contains the metal has additional benefits.

First, the organoaluminium gas is available in gas containers, allowing long life and easy replacement. The organoaluminium gas also effectively creates single charged aluminum ions. Finally, the halogens in the organoaluminium gas also act as an etchant to keep aluminum from accumulating on the walls of the arc chamber 100.

Second, the use of a vaporizer 190 is well known and the amount of vapor that introduced into the arc chamber 100 may be tightly controlled based on the temperature of the vaporizer 190.

Thus, this ion source is capable of producing single charged or multicharged ions of a selected species, where that species is a metal, such as aluminum, gallium, indium or lanthanum.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein. 

1. An indirectly heated cathode ion source, comprising: an arc chamber, comprising a plurality of walls; an indirectly heated cathode disposed in the arc chamber; a vaporizer in communication with the arc chamber; a heater to heat dopant material disposed within the vaporizer; a first valve in communication with the arc chamber and a first gas source; a second valve in communication with the arc chamber and a second gas source; and a controller in communication with the first valve, the second valve and the heater so as to operate the indirectly heated cathode ion source in one of a plurality of modes.
 2. The indirectly heated cathode ion source of claim 1, wherein the plurality of modes comprises two single charge modes to create ions of a species having a single charge and a multicharge mode to create ions of the species having two or more charges.
 3. The indirectly heated cathode ion source of claim 2, wherein the species comprises a metal.
 4. The indirectly heated cathode ion source of claim 2, wherein in a first single charge mode, the controller actuates the heater, opens the first valve and closes the second valve.
 5. The indirectly heated cathode ion source of claim 2, wherein in a second single charge mode, the controller disables the heater, and opens the second valve.
 6. The indirectly heated cathode ion source of claim 5, wherein in the second single charge mode, the controller opens the first valve.
 7. The indirectly heated cathode ion source of claim 2, wherein in the multicharge mode, the controller actuates the heater, and opens the second valve.
 8. The indirectly heated cathode ion source of claim 1, wherein the dopant material disposed in the vaporizer comprises a solid compound comprising a metal and the metal is also a component of a second gas contained in the second gas source.
 9. The indirectly heated cathode ion source of claim 8, wherein the metal is aluminum and the dopant material disposed in the vaporizer is aluminum chloride.
 10. The indirectly heated cathode ion source of claim 8, wherein the metal is aluminum and the dopant material disposed in the vaporizer is aluminum iodide.
 11. The indirectly heated cathode ion source of claim 8, wherein the metal is aluminum and the first gas source contains an inert gas and the second gas source contains dimethylaluminum chloride or trimethylaluminum chloride.
 12. A method of operating an indirectly heated cathode ion source in a plurality of modes, wherein the indirectly heated cathode ion source comprises a controller, an arc chamber, a vaporizer in communication with the arc chamber, and a heater to heat dopant material in the vaporizer, the method comprising: selecting a desired mode of operation; and using the controller to configure the indirectly heated cathode ion source to operate in the desired mode, wherein to operate in a multicharge mode, wherein the multicharge mode is used to create ions of a species having two or more charges, the heater is actuated so that vaporized dopant material comprising a metal enters the arc chamber and the controller enables a flow of a gas containing the metal into the arc chamber; wherein in a first single charge mode, wherein the first single charge mode is used to create ions of the species having a single charge, the controller enables the heater and enables the flow of an inert gas into the arc chamber; and wherein to operate in a second single charge mode, wherein the second single charge mode is used to create ions of the species having a single charge, the controller disables the heater and uses the gas containing the metal to generate a plasma.
 13. The method of claim 12, wherein the metal is aluminum and the dopant material disposed in the vaporizer is aluminum chloride.
 14. The method of claim 12, wherein the metal is aluminum and the dopant material disposed in the vaporizer is aluminum iodide.
 15. The method of claim 12, wherein the metal is aluminum and the gas containing the metal comprises DMAC or TMAC.
 16. An indirectly heated cathode ion source, comprising: an arc chamber, comprising a plurality of walls; a vaporizer in communication with the arc chamber; an indirectly heated cathode disposed in the arc chamber, wherein the indirectly heated cathode is used to generate a plasma in the arc chamber; and a controller configured to operate the indirectly heated cathode ion source in one of a plurality of modes, wherein in multicharge mode, the controller configures the indirectly heated cathode ion source such that two sources of a metal are used in a generation of a plasma.
 17. The indirectly heated cathode ion source of claim 16, wherein the metal is aluminum, material disposed in the vaporizer comprises aluminum chloride and wherein a gas containing aluminum is introduced into the arc chamber when operating in the multicharge mode.
 18. The indirectly heated cathode ion source of claim 16, wherein the metal is aluminum, material disposed in the vaporizer comprises aluminum iodide and wherein a gas containing aluminum is introduced into the arc chamber when operating in the multicharge mode.
 19. The indirectly heated cathode ion source of claim 16, wherein in a single charge mode, only one of the two sources is introduced into the arc chamber. 